1. Field of the Invention
This invention relates generally to the field of fabrication processing of flat panel imaging detectors and more particularly to the use of one or more shorting elements employing conductive materials in the underlying process for electrostatic discharge protection between components, the shorting elements removed during standard forming processes for the detector.
2. Description of the Related Art
Photosensitive element arrays for converting incident radiant energy into an electrical signal are commonly used in imaging applications, for example, in x-ray imagers and facsimile device arrays. Hydrogenated amorphous silicon (a-Si:H) and alloys of a-Si are commonly used in the fabrication of photosensitive elements for such arrays due to the advantageous characteristics of a-Si and the relative ease of fabrication. In particular, photosensitive elements, such as photodiodes, can be formed from such materials in conjunction with necessary control or switching elements, such as thin film transistors (TFTs), in relatively large arrays.
X-ray imagers, for example, are formed on a substantially flat substrate, typically glass. The imager includes an array of pixels with light-sensitive imaging elements, typically photodiodes, each of which has an associated switching element, such as a TFT or one or more additional addressing diodes. In conjunction with a scintillator, x-rays are transformed into visible light for imaging with the photosensitive elements. The photosensitive elements, typically photodiodes, are connected at one surface to a switching device, typically a TFT, and at the other surface to a common electrode which contacts all the photodiodes in parallel. The array is addressed by a plurality of row and column address lines having contact pads located along the sides of the array. In operation, the voltage on the row lines, and hence the TFTs, are switched on in turn, allowing the charge on that scanned line's photodiodes to be read out via the column address lines, which are connected to external amplifiers. The row address lines are commonly referred to as “scan lines” and the column address lines are referred to as “data lines.” The address lines are electrically contiguous with contact fingers which extend from the active region toward the edges of the substrate, where they are in turn electrically connected to contact pads. Connection to external scan line drive and data line read out circuitry is made via the contact pads.
As with most microcircuit devices, the elements of these arrays are subject to damage by electrostatic discharge (ESD). This is particularly true where the relative size, length and separation of traces may result in smaller relative capacitance values. To provide protection from ESD, prior art circuits have employed sacrificial capacitors to absorb ESD energy and therefore protect the image array from damage. But damaged capacitors generate a hard short between traces that needs to be completely removed by laser repair or an additional process step. Some of the known prior art devices use a TFT as an ESD protection device. The gate of the TFT is connected to drain electrode, so large static voltage will turn on the transistor and leak the static charge away before causing any damage. The on-resistance of the TFT is normally larger than 500 KΩ, for an average line capacitance such as 50 pf, 100 ns is needed to completely discharge the static charge. ESD damage such as oxide and nitride breakdown can occur within 10 ns. Therefore, TFT type ESD protection devices are only effective for relatively slow charge building up.
The most effective ESD protection method in the prior art is simply to connect all metal traces together to the ground, so there would not be any bias voltage build-up between them. Metal traces are later separated by using laser or mechanical glass scriber to allow panel testing or assembly. However, laser cutting inevitably generates conductive debris or particles. Mechanical scribing through metal traces will itself introduce ESD risk during scribing. It is also possible to separate metal traces from ground by using wet or dry etching processes. But that requires an additional process step.
It is therefore desirable to provide ESD protection for microcircuit elements during processing wherein the protection device is eliminated during processing without additional processing steps. It is further desirable that no debris or other contaminates from the protection device be present to contaminate processes elements or impart residual features on the final product which could impact performance.